Methods:The electron source in MCNP is simplified to be monoenergetic. The simulated PDD is first validated to measured PDD. The eyelid, Al backscatter cap, tungsten shield, and eyeball are stacked along the electron beam direction. Additional cap thickness of 1.5 mm and 2 mm are simulated to increase the thickness range. For comparison, 3 mm thick lead (Pb) shield is simulated. All dose values are normalized to the dose at dmax without the eye shield.

Results:Without any Al cap, the eyelid dose is 123%, 123%, 126% for 2 mm W, 3 mm W, and 3 mm Pb shields. With a 1 mm Al cap, the eyelid dose is 112%, 112%, and 114% respectively. With a 2 mm Al cap, the eyelid dose falls to 105% for all three shields. The backscattered photon dose component is <1% for all simulation scenarios so the backscatter dose is dominated by electrons. The transmission through the shield is <0.8%, suggesting the shield thickness can be reduced for 6 MeV electrons. Further study is needed for higher electron energy beams.

Conclusion:Of practical guidance value: If 110% or less hot spot is desired for eyelid, the 0.5 mm Al cap is not sufficient for 6 MeV. The 1 mm cap or 1.5 mm (0.5+1 mm) should be considered (while factoring in patient discomfort in using a thick cap).